The present invention is directed to antenna/filter combiners for connecting a plurality of frequency channels to a transmission antenna.
In mobile telephone systems, high power signals that are allocated to extremely narrow band frequency channels must be combined free of reaction or cross-talk and supplied to the transmission antennas in base stations. The plurality of frequency channels generally should be modularly expandable depending on the configuration level and, on the other hand, each transmission filter must be capable of being at least briefly tuned to every declared channel frequency during operation, without loss of signal quality.
High-Q resonators can be considered especially beneficial as low loss transmission filters because of the operation of current mobile radio telephone systems in the giga Hertz (GHz) range and because of the extremely narrow channel bandwidths used. The spatial expanse of such a system, however, only allows an ideal electrical combination of the transmission filters directly and punctiform (i.e., having the form or character of a point) at their outputs that is desired in the sense of a diplexer with what in part are substantial disturbances.
For different reasons, known solutions for diplexers in the microwave frequency range do not optimally solve the disclosed problem. For example, the article "Base Station Multicoupler Design for UK Cellular Radio Systems" by S. Kazeminejad, et al., which appeared in "Electronic Letters", 16 Jul. 1987, Vol. 23, November 15, page 812, proposed that the connection points of two transmission filters to an antenna line be provided with a half wavelength spacing therebetween. This spacing is fixed and is dependent on the frequency position of the filter. It would have to change correspondingly in a non-accomplishable way given signal echoing or returning of the filters. The clear function of the diplexer is, therefore, not assured given the interconnection of a plurality of transmission filters in this manner. Over and above this, the resulting structure is bulky and modular expandability is questionable.
Waveguide branching of separating filters is disclosed, for example, in the article "Computer-Aided Design of Wave Guide Multiplexers" by A. E. Atia, which appeared in "IEEE Transactions on Microwave Theory and Techniques", March 1974, pages 332-336. These filters have proven themselves in satellite and radio link technology. The filters are coupled to a waveguide shorted at the end, whereby, however, the spacing between the coupling planes (or levels) of the filters depend on their frequency positions, and, thus, are electrically significant and would likewise have to assume different values given filter echoing or returning in the separating filter. Additionally, the waveguide dimensions are relatively large for mobile radio telephone systems that currently operate between 0.9 GHz and 1.8 GHz and thus do not allow for compact structuring of the separating filters.
Channel branching filters with circulators have been utilized in radio link technology, as is known, for example, from the article "Channel Branching Filters for Wide Band Radio Relay Systems" by G. Ensslin, et al., which appeared in Telecom Report 10 (1987), Special "Radio Communication", pages 146-151. Modular structuring would appear possible with these filters. However, due to the relatively great number of circulators in the mechanical arrangement in the chain of channel branching filters--one circulator being allocated to each individual filter--many circulator passes and, thus, high signal attenuation results for every individual frequency channel. And high costs are presented by the use of so many circulators so that this solution cannot be considered as an optimum solution for purposes of mobile radio telephone systems.